The present invention relates to charge coupled devices, particularly to a process for forming buried electrodes, and more particularly to a three dimensional charged coupled device using buried electrodes which produce increased spectral range and has thick substrate back illumination, large full well and dynamic range capabilities, and radiation tolerance.
Charge coupled devices (CCD) were initially developed for an electrical analog to magnetic bubble memory. See Boyle et al., xe2x80x9cCharge Coupled Semiconductor Devicesxe2x80x9d, Bell Syst. Tech. Jour. 49, 593-600 (1970). Their analog charge handling capability made them useful in applications other than digital memory storage. Since their development, CCDs have been used to build analog delay lines, transversal filters, Fourier correlators, and signal processors. However, the greatest success has been their use as solid-state image sensors.
CCDs consist of closely spaced metal-oxide-semiconductor (MOS) capacitors located on the surface of a semiconductor. With appropriate dopant concentrations and capacitor electrode voltages, a space charge region is formed within the semiconductor directly below the surface of the MOS capacitor. This space charge region generates a potential well that stores charge generated with the material. This charge is generated by a variety of sources, from thermal electrons to injection via the photoelectric effect from photons that are absorbed within the semiconductor. When the voltages of the top electrodes of the MOS capacitors are pulsed in proper sequence, the potential wells move, transporting the stored charge from one MOS capacitor to the next. In this way the CCD becomes an image sensor capable of detecting, storing, and transporting charge generated by incident photons. This powerful concept has not changed over the past decades and the architecture used to implement this concept has changed very little.
CCDs are now being used in a wide range of image sensing applications, requiring different CCD topologies. The standard front illuminated CCD, see FIG. 1A, is useful for image sensing in the visible photon energy spectrum and recently in the 1-5 keV x-ray regime. These devices find use in video camera systems and facsimile and image reproduction equipment. Unfortunately this CCD architecture cannot be used in a variety of scientific and industrial applications which require imaging in the blue, ultra-violet, and soft x-ray energy spectrum. This is due to the fact that photons within these spectra are absorbed at the top electrode layer, which is no longer transparent at these energies.
To overcome this problem, a thinned, back illuminated CCD was introduced, see FIG. 1B. See xe2x80x9cThinned, Back Illuminated CCDs for Short Wavelength Applicationsxe2x80x9d, Tektronix Tech. Note July (1991). The problem of electrode absorption is overcome by turning the CCD upside-down and illuminating the backside of the substrate material. Unfortunately, the E-field generated by the topside electrodes is not strong enough to reach through to the backside of a standard device, as shown in FIG. 1A. Therefore, the substrate material is thinned from the backside until the E-field can reach through, typically 10-20 xcexcm (see FIG. 1B). Though high quality thinned, back illuminated devices are fabricated in this manner they are expensive and extremely fragile.
Image sensing and photon counting at higher x-ray energies from 1 keV to 30 keV, is an expanding market for CCDs. See Flint, xe2x80x9cCCD X-ray Detectionxe2x80x9d, EEV Tech Note November (1991). To provide this energy range, CCDs are being developed using a thick epitaxial, deep depletion layer or region with a relatively thin substrate, see FIG. 1C. Unfortunately, there is still a problem of collecting charge deep from within the substrate, generated by the high energy x-ray photons. A 100 xcexcm to 300 xcexcm thick, high resistivity epitaxial layer is required to extend the E-field into the semiconductor. Yet such epitaxial layers are difficult to fabricate, see Flint supra, and the E-field at these depths is still too weak to overcome any lateral movement of the generated charge caused by diffusion. These devices also suffer from charge spreading at higher photon energies. The charge will drift towards the semiconductor surface at an angle dependent upon its diffusion direction. Eventually the charge will be captured by the strong E-field near the surface, though this may occur several pixels away from its point of origin. The result is a loss of resolution caused by this charge spreading phenomena.
The problems outlined above are overcome by the present invention, a three dimensional CCD (3D-CCD) capable of developing a strong E-field throughout the depth of the semiconductor by using deep (buried) parallel (bulk) electrodes in the substrate material. Using backside illumination, the 3D-CCD architecture enables a single device to image photon energies from the visible, to the ultra-violet and soft x-ray, and out to higher energy x-rays of 30 keV and beyond. In the 3D-CCD, charge is transferred from bulk electrode to bulk electrode within the body of the substrate using bulk mode operation. This mode of charge transport within a semiconductor is ideal since the charge never comes in contact with charge traps located at the surface. See White, xe2x80x9cCharge Transport Without Trapsxe2x80x9d, Solid State Imaging, Proceedings of the NATO Advanced Study Institute on Solid State Imaging, pp. 275-294, September (1975). Thus, the 3D-CCD of this invention uses the entire bulk of the semiconductor for charge generation, storage, and transfer, and thus is a vast improvement over current CCD architectures that primarily use only the surface of the semiconductor substrate.
It is an object of this invention to provide a charge coupled device that uses the entire bulk of the semiconductor for charge generation, storage, and transfer.
A further object of the invention is to provide a three dimensional charge coupled device (3D-CCD) which utilizes deep (buried) parallel electrodes in the substrate material.
A further object of the invention is to provide a process for fabricating deep (buried) electrodes to create a 3D-CCD.
Another object of the invention is to provide a 3D-CCD capable of developing a strong E-field throughout the depth of the semiconductor material by using deep parallel electrodes in the material.
Another object of the invention is to provide a 3D-CCD which can image photon energies from the visible, to the ultra-violet and soft x-ray, out to higher energy x-rays of 30 keV and beyond.
Another object of the invention is to provide a 3D-CCD wherein deep parallel bulk electrodes are electrically connected to the surface electrodes, and an E-field parallel to the surface is established within the pixel.
Another object of the invention is to provide a 3D-CCD using deep parallel bulk electrodes, and wherein the E-field attracts charge to the bulk electrodes independent of depth, and confines it within the pixel in which it is generated, whereby charge diffusion is greatly reduced.
Another object of the invention is to provide a 3D-CCD fabrication process which includes utilizing chemical and/or plasma etching to form deep parallel electrodes in the substrate material.
Other objects and advantages of the present invention will become apparent from the following description and accompanying drawings. The invention is a new structure for CCDs which involves monolithic three dimensional 3D-CCD and a process for forming electrode structures within the bulk of a semiconductor. By use of chemical and/or plasma etching of the semiconductor materials, such as silicon (Si), the 3D-CCD structures use the entire bulk of the semiconductor material for charge generation, storage, and transfer due to the deep (buried) electrodes. Thus, the 3D-CCD of this invention provides a vast improvement over current CCD structures that primarily use only the surface of the semiconductor material or substrate. The 3D-CCD offers imaging capabilities for industrial, scientific, and military uses by greatly improving the performance of the CCD architecture or structures. Specific advantages of the 3D-CCD, when used as an image sensor, includes: 1) a very wide spectral response, 2) a very large full well capacity, 3) a very large signal to noise ratio, and 4) a high tolerance to radiation damage (built in antiblooming). The 3D-CCD utilizes deep (buried) parallel bulk electrodes which are formed in the substrate or semiconductor material. These deep bulk electrodes are electrically connected to the surface electrodes, and an E-field parallel to the surface is established. with the pixel, which for example includes four (4) deep parallel electrodes. This E-field attracts charge to the bulk electrodes independent of depth, and confines it within the pixel in which it was generated. Charge diffusion is greatly reduced because the E-field is strong due to the proximity of the bulk electrodes. Using backside illumination, the 3D-CCD structure enables a single device to image photon energies from the visible, to the ultra-violet and soft x-rays, and out to high energy x-rays of 30 keV and beyond. The operation of the 3D-CCD is similar to that of standard CCDs. There is no special voltage or clock waveform requirements, and the principles of charge transfer and storage are identical. In the 3D-CCD, charge is transferred from bulk or buried electrode to bulk or buried electrode within the body of the substrate using bulk mode operation. This mode of charge transport within a semiconductor is ideal since the charge never comes in contact with charge traps located at the surface. See White, referenced above.
Fabrication of the 3D-CCD basically involves forming an array of holes in a substrate where the bulk electrodes are to be formed, whereafter the surfaces or walls of the holes are doped and metalized, thereby bulk electrodes are formed in the substrate, and may for example have a diameter of 10 xcexcm and a depth of 200 xcexcm. From this point the formation of surface electrode placement is carried out using standard techniques to form the surface electrode structure and active output devices as in a standard CCD fabrication process.